1. The Field of the Invention
The invention relates to a passive component comprising a first track of a first material which is built up from at least a first and a second atom type and which knows a first and a second state.
2. Description of the Related Art
Such a component is known from Green et al., IBM Techn. Disc. Bull. 24 (1982), 5466. The known component is a resistor which has a substrate with a patterned layer of Si72Cr28, the indices indicating atom percents. The resistivity of this material is reduced by 40 to 50% through heating. A disadvantage of the known component is that the value cannot be substantially continuously adjusted over a range of at least a factor five.
It is an object of the invention to provide a passive component which has a continuously adjustable value. The object is achieved in that a first material has a first atom type and a second atom type, wherein the first material in a first state has an amorphous structure, the first material in a second state has a crystalline structure, and the second atom type is chosen from the group of gallium and germanium.
In the passive component according to the invention, the first material in the second state has a resistivity which is at least ten times, and preferably more than 1000 times lower than in the first state. In addition, the first material knows states with different structures which are each stable. A first portion of the first track is brought to at least a transition temperature through local heating, at which temperature the first material in the first portion crystallizes. Said first portion is smaller or greater in dependence on the value to which the component is to be adjusted. The value of the component is substantially continuously adjustable thereby. The value is in addition adjustable over a range equal to a factor five.
It was surprisingly found in experiments which led to the invention that the first atom type of the first material may be chosen from among various atom types, and that the second atom type may be chosen to be gallium, germanium, and indiumxe2x80x94also referred to as Ga, Ge, and In, respectively. Preferably, the proportion of the second atom type in the first material is at least 10%. Examples of first materials are Alxe2x80x94Ge, Gexe2x80x94Te, Gaxe2x80x94Sb, and Inxe2x80x94Sb. If silicon is chosen as the second atom type, a slight drop in the resistivity was found, such as in the case of Crxe2x80x94Si, or the first state is found to be not stable, such as in the case of Alxe2x80x94Si.
It may be that the first material comprises a third atom type. The inclusion of the third atom type in the first material renders it possible to increase the resistivity in the first state.
Heating to at least the transition temperature may be achieved inter alia by means of an electron beam, a needle-shaped heating source, a focused light beam, and a laser beam. The transition temperature generally lies between 100 and 400xc2x0 C., depending on the composition of the first material. Heating is preferably performed in that the layer is locally irradiated with a laser beam. The use of a laser beam has among its advantages that dimensions of patterns can be small. Moreover, a laser beam is a known tool, and laser ablation of the first material is also possible with a laser beam.
It is a first advantage of the component according to the invention that the value of the passive component according to the invention may be adjusted also after the manufacture of the component. This is advantageous because the value of the passive component can thus be adapted to the circumstances in which the component is used.
It is a second advantage of the component according to the invention that portions of the first layer can be easily removed. Removal may take place, for example, through local heating, for example with a laser beam. The possibility of removing portions of the first layer has among its advantages that the value of the component can be restored to its original value again. As a result of this, the component may be used, for example, as a potentiometer. The removal possibility has the further advantage that, should the value have been inadvertently adjusted to a value below the desired value, this value can be corrected in upward direction again. The component would have become useless without this possibility.
It is favorable when the first atom type is aluminum and the second atom type is germanium, and the first material has a germanium content of at least 20%. The aluminum-germanium material in the second state has a dual-phase, crystalline structure; separation of the aluminum and the germanium takes place upon crystallization. It was found from experiments that the difference in resistivity between the first and the second state is very small for a germanium content of less than 20%. Examples of favorable compositions of the aluminum-germanium are shown in Table 1.
A first advantage of aluminum-germanium is that the transition temperature lies between 80 and 320xc2x0 C. This is a temperature which can be easily achieved by local heating with a laser beam. At the same time, this temperature is sufficiently high for preventing a transition from the amorphous to the crystalline state under the influence of ambient factors. A second advantage of aluminum-germanium is that the ratio between the resistivity in the amorphous state and the resistivity in the crystalline state is usually more than 103. A third advantage of aluminum-germanium is that the atom types aluminum and germanium are non-toxic and are accepted in clean-room conditions.
The aluminum-germanium material is known from Catalina et al., Thin Solid Films 167 (1988), 57-65. This article, however, does not show or suggest the use of the material in a passive component, neither as a resistor, nor as a capacitor or as an integrated passive component. The article contains no suggestion for local heating of the material.
Alternatively, the first atom type is antimony. This is also referred to as Sb hereinafter. There are various first materials with antimony as the first atom type, such as Gaxe2x80x94Sb and Gexe2x80x94Sbxe2x80x94Te. The resistivity in the first state is high in these materials. In addition, the difference in resistivity between the first and the second state is satisfactory to great, i.e. from approximately a factor of 104 to 106. The value of the component can be adjusted over a wide range by means of this difference in resistivity. It is a further advantage of antimony as the first atom type that the first material can be restored from the second state to the first state. The first material will melt when irradiated with high-intensity laser light on a small surface area. The released heat diffuses so quickly then that the first material returns to the first state. Resistivity ratios between the first, amorphous state and the second, crystalline state are given in Table 1 for various materials.
In an embodiment of the passive component according to the invention, the first track is present on a first side of a layer of dielectric material, on a second side of which an electrode is situated. The passive component in this embodiment is a capacitor with a trimmable capacitance. The first track then constitutes or forms part of a second electrode, in as far as the first material is in the second state in the first track. The second electrode may comprise a strip of electrically conducting material. A transition from the first to the second state achieved in the first track will enlarge the surface area of the second electrode, so that the capacitor will have a higher capacitance.
In a first, more specific embodiment, the first track is an interconnect between a first and a second strip of electrically conducting material. Preferably, the first and the second strip have a large surface area as compared with the first track. If the first track is in the first state, it is only the first conductive strip which forms part of the second electrode of the capacitor. An electrically conducting connection between the first and the second strip is made in that the transition from the first to the second state is achieved in the first material. The second strip is connected, whereby the capacitance of the capacitor is increased.
In a second specific embodiment, the first electrode and the second electrode of the capacitor each have finger-type portions which lie side by side. The two electrodes are xe2x80x9cinterdigitatedxe2x80x9d, such that two mutually opposed sides of a finger-type portion adjoin the dielectric. The capacitance of this electrode can be quickly increased in that one or several finger-type portions of the second electrode comprising the first material is/are heated.
In an alternative embodiment, the passive component according to the invention is a resistor. It is an advantage of this embodiment that the resistance value can be adjustedxe2x80x94or trimmedxe2x80x94while it forms part of an electric circuit in operation. This is favorable for an accurate attunement of the resistance value to the circuit. At the start of trimming of the resistor according to the invention, a weak current flows through the circuit. If the component were trimmable to higher values only, as in conventional trimmable resistors, the current at the start of trimming would be high. This would involve a risk of a component of the circuit being destroyed. This risk is avoided in the component according to the invention used as a resistor.
It is favorable with the use of a passive component according to the invention as a resistor when the first track is present on an electrically insulating substrate on which furthermore a resistance track is present made of a second material with a resistivity lower than that of the first material in its first state, and said first track forms a bridge between a first and a second point on the resistance track.
If only the first material is present, this material will define the resistance value. This has among its disadvantages that the range of resistance values is limited by the properties of the first material and that trimming has to be performed very accurately in order to achieve low resistance values. The presence of a resistance track of a second material electrically parallel to the first track counteracts these disadvantages. The resistance track is short-circuited between the two points in that the first material of the first track is brought into its second state in this embodiment. This effective shortening of the resistance track determines the reduction in the resistance value of the component; the reduction is not primarily determined by the material used and the dimensions of the resistance track, as is the case in the resistance of Green et al.
It is furthermore an advantage of this embodiment that damage and pollution of at least the first track and the resistance track are prevented. Material is burnt away from the first track, thus creating a cut, during trimming in components which are trimmable to higher resistance values in a conventional manner. The cut may expand, whereby the stability of the resistor is affected. It is also possible for the burnt-away material upon its escape to damage a layer which covers the first track. Furthermore, the burnt-away material may deposit on the first track and pollute this track in an uncontrollable manner. These disadvantages are avoided in the embodiment of the passive component according to the invention.
The choice of the second material is one of the determining factors for the range over which the resistance value of the component according to the invention is tunable. If a resistivity of approximately 200 xcexcxcexa9cm is chosen for the resistance track, tungsten-titanium-nitrogen is preferably chosen. The resistance track may be manufactured in a sputtering process in which nitrogen is added to tungsten-titanium. It is an advantage of this material that it need not be annealed for guaranteeing a low temperature dependence of the resistivity. The resistivity of the material has a comparatively low temperature coefficient. As a result of this, it can be provided together with the first material in one sputtering process. In addition, the material is insensitive to the etching liquid with which the first material can be patterned into the first track and any other patterns which may be present. The use of tungsten-titanium-nitrogen and of aluminum-germanium in the resistor according to the invention is furthermore favorable because tungsten-titanium-nitrogen can be etched in an etchant which does not attack aluminum-germanium. Aluminum-germanium can be etched in an etchant which does not attack tungsten-titanium-nitrogen, such as a mixture of iron-trichloride hexahydrate and hydrogen chloride.
If a lower resistance than 200 xcexcxcexa9cm is chosen for the resistance track, CuNi is a suitable second material, having a resistivity value of approximately 40 xcexcxcexa9cm. Again, this material need not be annealed for obtaining a low temperature coefficient. If a higher resistance value is chosen, CrSiN is suitable as a first material, with a resistivity of 2xc3x97103 xcexcxcexa9cm. This material does have to be annealed for obtaining a low temperature coefficient of the resistivity.
In a favorable modification of the embodiment of the passive component according to the invention, the resistance track extends from a first to a second electrode, and the resistance track is substantially meandering in shape. A meandering resistance track has among its advantages that bridges made of the first material may be positioned in a large number of points on the resistance track. Also, a substantially meandering resistance track preferably comprises a first, wider strip of the second material. The passive component can be trimmed to higher resistance values in a conventional manner by means of such a strip, for example in that material is removed with a laser beam.
The resistance can be trimmed stepwise to a lower value if not only a first, but also a second and possibly a third bridge comprising the first material is/are provided between several loops of the meandering resistance track. In an advantageous modification of the embodiment of the component according to the invention, a bridge comprising the first material is present between a third and a fourth point on the resistance track, which third and fourth point lie between the first and the second point of the resistance track. In this modification, the bridge between the third and the fourth point may be first locally heated, and after that the bridge between the first and the second point, if so desired. The step size of the trimming process is thus reduced, which enhances the accuracy of the adjustability of the value of the component.
In a further modification of the embodiment of the passive component according to the invention, the resistance track has the shape of a loop between a fifth and a sixth point. The resistance track thus comprises a first region which is substantially covered by a layer of the first material. Such a loop may have, for example, a V-shape, a U-shape, a hairpin shape, or form the arc of a circle or oval. The value can be trimmed over a continuous scale in this modification.
Preferably, the first track has the shape of a narrow strip which is narrower than the laser beam to be used for the local heating. This shape is obtained in that a layer of the first material is provided and this layer is subsequently patterned by etching. If the ratio of the resistivity values of the first material in the first and the second state is above 1000, it is not necessary to pattern the first layer. The leakage current through the first material in its first state can be small then. It may be favorable that first the, possibly patterned, layer of the first material is present on the substrate, with the resistance track of the second material disposed thereon.
The passive component according to the invention may furthermore be included in a network of passive components. Such a network of passive components is also known as a thin-film module and is described in the application EP1024535 (not-published), which is deemed to be included in the present application by reference. A thin-film module is suitable, for example, for improving the signal-to-noise ratio. It is furthermore highly suitable for contactless products and electronic appliances. The integration saves space and expense. It is advantageous also for a thin-film module that the resistance value and/or the capacitance value are still adjustable after the manufacture of the thin-film module has been completed. Thin-film modules often comprise various passive components which are to be attuned to the performance of a specific function. The passive component according to the invention renders it possible for the thin-film module to be corrected after its inclusion in an electric circuit. It is also possible with the passive component according to the invention to correct a possible production error owing to which the value of the module would lie outside the relevant tolerances. This is of especial importance for thin-film modules because of their high cost price.
In a particularly favourable embodiment the passive component is used as a switch in a system of batteries. Such batteries can be Li-ion batteries, for example, and are preferably used in a fuel-cell operated vehicle.